The ciliation of the endometrial epithelium in declining pseudopregnancy in the rabbit will be analyzed further. The origin of the ciliated cells from multinucleated precursors without the intervention of a mitotic process will be studied by autoradiography. The identification of the intracytoplasmic debris of ciliating cells as chromatin will be studied by the Feulgen technic. The nature of the interacting surfaces (trophoblast and endometrium) in the implantation process will be critically studied by transmission and scanning electron microscopy. The effect of synchronous and asynchronous interaction or lack of it between blastocyst and endometrium will be studied.